Junstrom was the bass guitarist of Lynyrd Skynyrd from its formation in 1964, until being replaced by Greg T. Walker in 1971. Donnie Van Zant, the younger brother of the Lynyrd Skynyrd leader, Ronnie Van Zant, formed .38 Special in 1974, with Junstrom joining as the bass guitarist in 1977.

An avid amateur radio operator, he has the call letters K4EB which translate to "Known 4 Excellent Bass".

Junstrom retired from .38 Special in 2014, due to an injury that required surgery.